ACTINOZOA. 



181 



an egg-covering, and with an eqnal degree of 

 rapidit}^ After yolk-cleavage the embryo appears 

 rudely cylindrical in form, a belt of cilia passing 

 round the middle of its body (^fig. 31). This soon 



Fig. 31. 





Development of Pleueobrachia : — a, newly extruded ovum, 

 showing the yolk with its external covering ; b, the same, after 

 segmentation ; c, d, and e, successive stages of the embryo. (All 

 magnified.) 



breaks up into two lateral groups which eventually 

 disappear altogether ; the ctenophores, at first very 

 broad and few in number, at an early period taking 

 on the performance of their special function. The 

 tentacles are at first destitute of lateral fringes, 

 traces of which appear while the rest of the canal 

 system is but imperfectly indicated. The form of 

 the adult animal is, according to Agassiz, fully 

 assumed before the young organism attains a 

 length of '04 of an inch. 



N 3 



